This invention relates to a powder material supply device which is used for producing a predetermined form of objects by supplying powder material to a mold system. This invention also relates to a production method of a fuel cell separator and a fuel cell separator produced by this production method using the powder material supply device. Here, the term xe2x80x9cpowder materialxe2x80x9d generally refers to material in the forms of powder, particle and/or short fiber.
A fuel cell which generates electric power by making use of fuel gas and oxidant gas, especially a solid polymer type fuel cell, is considered to be a new clean energy source in various applications including automobiles. A solid polymer fuel cell is configured in such a way that an ion conductive solid electrolyte membrane is sandwiched by an anode and a cathode each having a catalyst and functioning as a gas diffusion electrode, and an outside of each electrode is further provided with a fuel cell separator. The separator at the anode provides hydrogen as fuel gas, and the separator at the cathode provides oxygen as oxidizer gas.
FIGS. 5(a) and 5(b) show an example of such a fuel cell separator. As shown in FIGS. 5(a) and 5(b), on a fuel cell separator 1, narrow grooves 1a are formed on a planar surface thereof. In order to increase an overall surface area for contacting between the gas diffusion electrode and the gas, the grooves 1a are meandering with a small pitch on the whole surface of the separator. The grooves 1a may be formed on both surfaces of the separator as shown in FIG. 5(b), or may be formed only on one surface of the separator.
There are other types of structures of the separator wherein both surfaces or one surface is provided with a large number of projections where spaces between those projections are used as passages of the gas, or both surfaces or one surface is provided with combinations of such projections and grooves.
In the fuel cell separator described above, the following characteristics are required.
(1) Gas non-permeability. This is a characteristic not to allow the hydrogen gas and/or oxygen gas supplied thereto to permeate through the separator. Generally, a fuel cell is formed of many cell units stacked together where each cell unit includes a solid polymer electrolyte membrane at the center, gas diffusion electrodes at both outer sides of the electrolyte membrane, and the separators further outside of both of the electrodes. The gas is flowing at least one side of a separator. Therefore, if the separator has gas permeability or leakage, an overall efficiency of power generation by the fuel cell decreases, or the power generation itself cannot be performed.
(2) Electric conductivity. Because the separator also acts as an electrode of the fuel cell, electric conductivity is an essential requirement.
(3) High surface or thickness accuracy. Because electric current flows through the contact area between the separator and the anode or cathode, insufficient surface accuracy, i.e, profile irregularity, deteriorates electric conductivity due to the insufficient contact area. Moreover, when the surface accuracy is insufficient, gaps may be formed between the anode and the cathode, which may cause breakage of the separator when a force is applied to the separator in such a direction to press the gaps. When the surface accuracy is high, the contact resistance is small, resulting in improvement of the fuel cell performance. The surface accuracy is represented by a difference d between the maximum thickness Tmax and the minimum thickness Tmin by measuring the thickness at predetermined points on the separator.
In order to satisfy the requirements noted above, in the conventional technology, the fuel cell separator is produced through a mechanical process such as machine works on a graphite board. However, due to the long processing time, the fuel cell separator becomes too expensive.
Recently, a production method using a mold technology has been employed for the purpose of producing the fuel cell separators. In this conventional method, powder material is formed by mixing carbon powder in the form of powder and/or short fiber and synthesis resin powder. The resultant powder material is supplied to a lower mold of a mold system such as a press machine which is then covered by an upper mold of the mold system. The fuel cell separator is formed by pressing the powder material by the press machine in the high temperature condition.
However, the conventional production method using the mold technology described above has a problem in that it is difficult to achieve the sufficient level of surface accuracy of the separator. For example, in this conventional production method, the difference d of the thickness described above tends to be 0.2 mm or more, thereby limiting the performance of the fuel cell.
The present invention reflects on the aforementioned facts, and aims to provide a fuel cell separator which is able to achieve the surface accuracy higher than that of the conventional technology, a production method of the fuel cell separator, and a powder material supply device suited for the production method.
In order to achieve the objectives above, the powder material supply device of the present invention is characterized in having a material supplier which is provided with a plurality of supply openings in a downward direction, a slide plate provided at the lower part of the material supplier and is capable of slidably moving between a location where all of the supply openings are closed and another location where all the supply openings are opened, and a base to support the material supplier and the slide plate.
The material supplier has a predetermined constant thickness or height, thereby forming the same capacity for each and every supply opening formed thereon. Preferably, engagement means are provided on the powder material supply device for accurately positioning the powder material supply device on a mold system, such as a lower mold to which the powder material is supplied. The supply openings are aligned on the material supplier equally and accurately in, for example, a matrix manner.
The production method of the present invention in which a fuel cell separator is formed by supplying the powder material to a lower mold and applying pressure and heat between the upper mold and the lower mold, is characterized in that the powder material for the fuel cell separator is supplied through a plurality of supply openings.
Further, the production method of the present invention in which a fuel cell separator is formed by supplying the powder material to the lower mold and applying pressure and heat between the upper mold and the lower mold, is characterized in that the powder material supply device having either one slide plate or two slide plates are used for supplying the powder material to the lower mold.
The fuel cell separator of the present invention is characterized as having a surface accuracy of less than 0.07 mm wherein the material of the fuel cell separator is a mixture of carbon powder and synthetic resin powder and the size of the separator is more than 200 mm by 200 mm.